1. Field of the Invention
The present invention relates to AC-DC power converters, and, more particularly, to the provision of a resonant power converter having harmonic neutralization that has application in battery chargers and power supplies for automotive, industrial, and a variety of consumer applications.
2. Description of the Related Art
Some electric loads cause the generation of harmonic currents on the power supply to which these loads are connected. These harmonic currents are undesirable as they cause non-optimal use of the power source by the loads. As a consequence, some European countries have imposed stringent requirements for the elimination of such harmonics, referred to herein as "harmonic neutralization", through the standard known as IEC 555-2. Other countries are contemplating the requirement for harmonic neutralization.
Certain types of electrical loads, such as electrical heating units, are inherently free from the generation of harmonic currents while others, such as power supply type loads, normally produce large harmonics of current. For example, when an ordinary AC input power converter is operated from the power source provided by the electric utility service, the power converter is likely to produce harmonic current. Specifically, though the power source has a sinusoidal voltage, such as the 60 Hz power source provided in the United States, the power converter connected to the sinusoidal voltage power source draws non-sinusoidal current. The current drawn by the power converter often has a distortion content which ranges from 25 to 150% thereby creating low-frequency conducted interference and unduly taxing the current supplying capacity of the power source.
When a power converter includes harmonic neutralization, the power converter ideally draws sinusoidal current from the sinusoidal voltage power source. The harmonic neutralizing power converter does not create low-frequency conducted interference, and, if the converter's input current is in phase with its input voltage, the converter provides optimum utilization of the power source. Under these circumstances, the power converter looks like a linear resistive load to the power source and the power factor of the converter is unity--the optimum.
The employment of harmonic neutralization is well known in the art in switchmode, i.e., in pulse-width modulation (PWM), power supplies. Several circuit topologies exist which, when used with PWM and suitable control loops, meet current harmonic neutralization standards. Some of the these topologies support outputs of several hundred watts. Industrial loads, ranging up to and beyond a kilowatt and which normally generate large harmonic currents, are sometimes supplied by power conditioning equipment employing active and/or passive harmonic neutralization circuits to meet harmonic neutralization standards. However, the use of PWM to achieve harmonic neutralization is limited from an implementation viewpoint in many respects. First, power converters using PWM for harmonic neutralization are costly to manufacture, usually require additional control of electromagnetic interference (EMI), are of significant size and weight, operate inefficiently at high frequencies, and adapt poorly to induction coupling. Induction coupling is desirable in some applications, such as in providing for easier, safer, and more reliable battery charging. The likelihood of the imposition of harmonic neutralization standards as in the United States and the retention of existing standards as in Europe makes it desirable to develop a power converter which does not possess the aforementioned limitations of PWM power supplies.
Resonant converters are advantageous over switchmode (PWM) converters for several reasons which are discussed hereinafter. Therefore, resonant converters may serve as a viable alternative to PWM converters if harmonic neutralization can be achieved with resonant converter topology. Though the basic concepts involving resonance in electrical circuits were developed during the early days of the development of radio technology over fifty years ago, the evolution of resonant technology has been generally limited to the resolution of specific problems, e.g. the commutation of silicon controlled rectifiers (SCRs). A brief look at the history of power supplies is instructional when comparing PWM converters to resonant converters.
Early power supplies often used a line frequency power transformer and a linear regulator consisting of vacuum tubes or, in later supplies, power transistors. These early power supplies were generally large, heavy and inefficient. DC--DC type power supplies used mechanical vibrators, vacuum tubes or switching power transistors to accomplish inversion (DC to AC transformation) when isolation or a significant voltage transformation was required. When vacuum tubes or switching transistors were employed, the regulation function of the power supply was often accomplished by PWM or by pulse-frequency modulation. The introduction of silicon power transistors, which were capable of dissipating several hundred watts and switching in a few microseconds, had an impact on the popularity of PWM for regulation. Today, PWM still prevails as the means for regulating a vast majority of commercially available power supplies.
The availability of high speed silicon power transistors resulted in the emergence of PWM power supplies having increasingly higher feasible power levels. However, these power supplies were generally limited to only a few kilowatts. In the late 1960's the SCR became available for use in operation at several kilowatts and above. However, because the SCR has no means by which it can interrupt its own current flow, i.e., self-commutate, forced commutation was necessary for the SCR to operate in DC systems. The need for forced commutation prompted the development of resonant circuits in which ringing is used to produce a reversal in current flow. Similarly, since the mid-1980's, resonant power technology is utilized with increased frequency in power circuits due to the introduction of other switching devices such as bipolar transistors, MOSFETs, IGBTs, etc. Though these devices, unlike SCRs, do not necessarily require the use of resonance for commutation, resonance can be employed where it is advantageous over PWM, such as at high operating frequencies.
When operated at the resonant frequency of its tank circuit (as used herein, the term "tank" refers to the combination of a transformer or inductor and the resonant capacitor connected thereto), the resonant converter has a pure sine wave of tank current at that frequency. Therefore, at the time the square wave of excitation voltage produces a voltage transition on its power switching devices, these devices, in resonant converters, are not necessarily conducting current. Low switching losses are produced in these devices when the current is zero. Thus this condition, commonly referred to as zero-current switching, is desirable. With resonant converters the dominant loss is conduction loss. However, conduction losses are not strongly related to operating frequency and, therefore, the resonant converter can operate efficiently at a high frequency. For example, a resonant converter can be designed to operate efficiently at a frequency that is typically five to ten times higher than a PWM converter of the same power level using the same power switching devices.
A resonant converter is further advantageous as it produces little electromagnetic interference (EMI). Because a resonant converter develops a sinusoid of current, as opposed to a fast rising quasi-square wave, little EMI is produced. Further, the components of the resonant converter are generally fewer in number and less costly than the components required for a PWM converter. The resonant converter's components are also of a smaller volume and weight thereby permitting applicability of the converter in a smaller or more weight sensitive environment than is possible with PWM. It is therefore desired to provide a power converter having harmonic neutralization which uses resonant power technology to gain the advantages inherently provided by resonant converters.
As previously mentioned, resonant converters are known to be advantageous for having low switching loss. However, there are problems associated with the use of resonant converters that should be considered. For example, switching losses may be created when the output of the resonant converter is controlled over a wide range. Specifically, the output voltage or current of a resonant converter is typically controlled by changing its operating frequency over a range of frequencies above or below the converter's resonant frequency. When the operating frequency is above or below the resonant frequency, the tank current is out of phase with the excitation voltage. Generally, this phase difference creates switching loss and may increase EMI as well. Specifically, such losses may occur over an operating frequency range which extends from several times the resonant frequency down to one-half (1/2) the resonant frequency.
Another problem occurs when the operating frequency is close to the resonant frequency. In such a situation, the voltage or current in the tank components is strongly related to the Q of the circuit. Thus, the control curves are highly nonlinear and strongly affected by the load. Therefore, it is desired to provide a controlled power converter using resonant power technology and having harmonic neutralization which avoids the problems encountered when the converter is controlled by changes to the converter's operating frequency.
Yet another potential problem associated with the resonant converter is the conductive losses that may be generated. A resonant converter may have significantly higher conduction loss than is generated with a PWM converter. However, if the switching losses of the resonant converter is minimized as described hereinabove, the resonant converter can still incur lower total loss than the PWM converter. Thus, the resonant converter's conductive losses do not preclude its desirability over the use of PWM converters.
As previously stated, the problem of switching loss for a resonant converter can be eliminated when the series resonant converter operates at a frequency below one-half of its resonant frequency. If the controlled switches of the resonant converter are turned off prior to the initiation of a second cycle of ringing, current in the tank circuit ceases to flow. This mode of operation of a resonant converter is referred to herein as the "discontinuous current mode of operation". When in the discontinuous current mode of operation, the resonant converter can have a constant on-time drive.
A series resonant converter not only has low switching loss in all of its switching components if operated at and/or below one-half of its resonant frequency, but is also advantageous over other resonant converters due to its low component count. Further, over an operating frequency range from zero to one-half of its resonant frequency, a series resonant converter's output current is nearly linearly proportional to its operating frequency, the tank current waveform is nearly independent of the repetition rate, and the average current output is proportional to the repetition rate.
When operating in the discontinuous current mode, the output current of a series resonant converter is quite independent of its output voltage. Specifically, from zero output voltage to an output voltage at which the input/output voltage transformation ratio is approximately unity (1.0), the output current of a series resonant converter operating in discontinuous current mode is nearly constant. At output voltages reaching a voltage transformation ratio of unity, the output current falls off rapidly as the output voltage exceeds the tank excitation voltage and the tank becomes unloaded. In addition to the provision of an essentially constant output current, the series resonant converter tolerates any passive load and needs no protection circuitry to limit its output voltage or current. Therefore, it is desirable to employ a series resonant converter topology for a power converter having harmonic neutralization to utilize the aforementioned advantages inherently provided with series resonant converters.
A series resonant topology is utilized in an AC to DC converter in U.S. Pat. No. 4,143,414 to result in a reduction in harmonic current distortion. Each phase of the three-phase AC source is first rectified by a full-wave bridge rectifier to convert the AC phase voltage to a rectified DC voltage. Each DC voltage is in turn provided to a resonant bridge inverter to invert the DC voltage to an AC voltage. Then, the AC voltage is converted by another full-wave rectifier to a full-wave rectified DC voltage. The combination of the series resonant bridge inverter and the second full-wave bridge rectifier behaves somewhat like a resistive load for the first full-wave rectifier to reduce the harmonic current distortion produced in the three-phase AC voltage source. However, though a reduction in harmonic distortion occurs, current harmonics are generated in each phase of the three-phase source because the DC to DC rectifiers (the combination of the series resonant bridge inverter and the second full-wave bridge rectifier) are non-linear and do not present an ideally resistive load. As a consequence, additional circuitry is used in the converter of U.S. Pat. No. 4,143,414 to eliminate the harmonic current generated by the DC to DC rectifiers.
Therefore, it is desired to develop a harmonic neutralizing power converter using series resonant topology which requires a minimal number of components to achieve neutralization and which does not generate its own harmonics which must be eliminated through the use of additional circuitry. Further, as previously discussed, it is desired that control of the converter be accomplished by means other than modulation of the operating frequency of the series resonant converter.